Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications. Ethylene glycol, commonly known as monoethylene glycol (MEG), is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
MEG is typically prepared from ethylene oxide, which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent stream containing mostly water. The ethylene oxide-depleted stream is partially or entirely supplied to a carbon dioxide absorption column wherein the carbon dioxide is at least partially absorbed by a recirculating absorbent stream. Gases that are not absorbed by the recirculating absorbent stream are recombined with any gases bypassing the carbon dioxide absorption column and are recycled to the ethylene oxide reactor.
The solvent stream leaving the ethylene oxide absorber is referred to as fat absorbent. The fat absorbent is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed from the fat absorbent as a vapour stream. The ethylene oxide-depleted solvent stream is referred to as lean absorbent and is recirculated to the ethylene oxide absorber to absorb further ethylene oxide.
The ethylene oxide can then be converted into MEG. In one well-known process, the ethylene oxide is reacted with a large excess of water in a non-catalytic process. This reaction typically produces a glycol product stream, after removal of water, consisting of almost 90 weight percent MEG, the remainder being predominantly diethylene glycol (DEG), some triethylene glycol (TEG) and a small amount of higher homologues. In another well-known process, ethylene oxide is reacted with a smaller excess of water in the presence of a hydrolysis catalyst. In a further well-known process, ethylene oxide is catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate can improve the selectivity to monoethylene glycol, although diethylene glycols and higher ethylene glycols, such as triethylene glycol and tetraethylene glycol (TTEG) will still be present in the produced glycol product stream.
The ethylene glycol-containing aqueous streams that result from the known processes are subjected to water removal, typically in a series of flashing and/or distillation steps. The water removal is an energy intensive process, particularly if a large excess of water is present in the ethylene glycol-containing streams. After water removal, further distillation steps are then required in order to purify the MEG and separate the DEG, TEG and higher homologues. Distillation of glycols at high temperature can lead to decomposition and the production of quantities of by-products. Therefore, the distillation of glycols is generally carried out under reduced pressure, further increasing the energy requirement for purification.
A process for the extraction of MEG and propylene glycol from aqueous streams using ionic liquids, particularly tetraethyl ammonium-2-methyl-1-naphthoate is described in both Ind. Eng. Chem. Res. 2013, 52, 4902 and Separation and Purification Technology, 2012, 97, 2. However, the use of such high-boiling ionic liquids will provide further issues relating to the purification of materials after liquid-liquid extraction. High-boiling impurities will accumulate in the ionic liquids on repeated use, reducing their potential recyclability. Further, ionic liquids tend to have high viscosities leading to handling and mixing problems.
The present inventors have sought to provide a process for the facile separation of alkylene glycols, particularly ethylene glycol in which the large distillation duty required in the water removal and subsequent purification of the remaining glycols is significantly reduced and in which materials are easily re-used in an industrial process.